It has been argued that global cities are those sharing the following
characteristics:

International, first-name familiarity; whereby
a city is recognised without the need for a political subdivision. For
example. although there are numerous cities and other political entities
with the name Paris or variations on it, one would say "Paris", not "Paris,
France". Active influence and participation in international
events and world affairs; for example, New York City is home to the
United Nations headquarters complex and consequently contains a vast
majority of the permanent missions to the UN[3]. A fairly large population (the centre of a
metropolitan area with a population of at least one million, typically
several million). A major international airport (for example,
London Heathrow Airport) that serves as an established hub for several
international airlines. An advanced transportation system that includes
several freeways and/or a large mass transit network offering multiple modes
of transportation (rapid transit, light rail, regional rail, ferry, or bus).
In the West, several international cultures and
communities (such as a Chinatown, a Little Italy, or other immigrant
communities). In other parts of the world, cities which attract large
foreign businesses and related expatriate communities; for example,
Singapore, Shanghai, Hong Kong, Tokyo, and Moscow. International financial institutions, law
firms, corporate headquarters (especially conglomerates), and stock
exchanges (for example the London Stock Exchange, the New York Stock
Exchange or the Tokyo Stock Exchange) that have influence over the world
economy. An advanced communications infrastructure on
which modern trans-national corporations rely, such as fiberoptics, Wi-Fi
networks, cellular phone services, and other high-speed lines of
communications. World-renowned cultural institutions, such as
museums and universities. A lively cultural scene, including film
festivals (for example the Toronto International Film Festival), premieres,
a thriving music or theatre scene (for example, West End theatre and
Broadway); an orchestra, an opera company, art galleries, and street
performers.
Several powerful and influential media outlets
with an international reach, such as the BBC, Associated Press, Reuters, The
New York Times, The Times, or Agence France-Presse.
A strong sporting community, including major
sports facilities, home teams in major league sports, and the ability and
historical experience to host international sporting events such as the
Olympic Games, Football World Cup, or Grand Slam tennis events.
To some, London, New York City, Paris, and Tokyo have been traditionally
considered the 'big four' world cities – not coincidentally, they also serve
as symbols of global capitalism. However, many people have their own
personal lists, and any two lists are likely to differ based on cultural
background, values, and experience.

In certain countries, the rise of suburbia and the ongoing migration of
manufacturing jobs to these countries has led to significant urban decay.
Therefore, to boost urban regeneration, tourism, and revenue, the goal of
building a "world-class" city has recently become an obsession with the
governments of some mid-size cities and their constituents.

The phenomenon of world-city building has also been observed in Buenos
Aires, Santiago, Frankfurt, Montréal, Sydney, Mexico City and Toronto: each
of these cities has emerged as large and influential.

GaWC Leading World Cities (2004 Edition)
An attempt to redefine and recategorise leading world cities was made by PJ
Taylor at GaWC in 2004.
This ranking list is referred to as the Official GaWC List.

Global Cities

Well rounded global cities
Very large contribution: London and New York City.
Smaller contribution and with cultural bias: Los Angeles, Paris and San
Francisco.
Incipient global cities: Amsterdam, Boston, Chicago, Madrid, Milan, Moscow,
Toronto.